The present invention relates to a frequency synthesizer, and more particularly, to a hybrid frequency synthesizer mixing a direct digital frequency synthesizer(DDS) and a phase locked loop(PLL) synthesizer in a wireless communication system.
FIG. 1 is a block diagram of the conventional hybrid frequency synthesizer.
Referring to FIG. 1, the conventional hybrid frequency synthesizer comprises a PLL B and a DDS A1. PLL B in the hybrid frequency synthesizer consists of a phase comparator 15 comparing a reference frequency and a frequency of a voltage controlled oscillator fed-back through N frequency divider and generating an error voltage corresponding to difference of phase and frequency, a low pass filter 16 for removing a fluctuation component of the error voltage, a voltage controlled oscillator 17 for changing an oscillating frequency according to a control voltage inputted through the low pass filter 16, and N frequency divider 18 for outputting 1/N signal frequency of signal frequency outputted from the voltage controlled oscillator 17.
Firstly, the phase comparator 15 compares an input reference frequency and the frequency of the voltage controlled oscillator oscillator 17, and N frequency divider becomes stable.
At this time, the output frequency of the voltage controlled oscillator is fixed to a specific frequency.
The conventional PLL frequency synthesizer fixes a reference frequency to a specific frequency, obtains an output frequency corresponding to N times of the reference frequency by changing a dividing ratio of the N frequency divider.
However, the hybrid frequency synthesizer of the DDS and the PLL changes a reference frequency provided from the DDS instead of changing the dividing ratio of the PLL in order to obtain the fine frequency resolution and fast setting time.
Referring to FIG. 1, the DDS A1 of the conventional hybrid frequency synthesizer consists of a phase accumulator 10 accumulating an instantaneous phase data K inputted as a digital type by each clock, a phase/amplitude converter 11 outputting a digital amplitude value of a sinewave with respect to the accumulated discrete phase value, a digital/analog converter 12 outputting an analog amplitude value of the sinewave, a low pass filter 13 removing harmonic components and sampling harmonic component of the analog amplitude value, and a limiter 14 converting the analog sine wave into a rectangular wave and providing the rectangular wave as a reference frequency of the PLL.
The operation of the DDS A1 used in the conventional hybrid frequency synthesizer will be described below.
The DDS A1 generates a sinewave, and converts the generated sinewave into a rectangular wave determined by the instantaneous phase data K, and provides the rectangular wave through a limiter to a reference frequency input terminal of the PLL synthesizer.
The DDS A1 is a method for accumulating an instantaneous phase value by each clock and synthesizing one period of the output frequency.
Accordingly, an arbitrary frequency is easily generated since the corresponding output frequency is generated each when the instantaneous phase value inputted as a type of a digital word is changed.
The output frequency of the DDS A1 forms one period each time the accumulated result of the phase accumulator overflows. Consequently, the most significant bit designates the output frequency.
However, the output of the most significant bit of the phase accumulator in the conventional DDS cannot be directly used as a reference frequency because it periodically changes its duty cycle, and generates jitters according to the input instantaneous phase value. So the hybrid synthesizer has a problem in that a spurious and phase noise level in the output of the PLL is increased when directly using the most significant bit as the reference frequency of the PLL.
Accordingly, the conventional hybrid frequency synthesizer depicted in FIG. 1 generates the reference frequency by converting the output of the phase accumulator to an analog sine wave in order to remove the jitter components with a phase/amplitude converter, a digital/analog converter, low pass filter, and a and finally obtains a rectangular reference frequency through the limiter. The limiter is placed between the DDS and the PLL in order to provide a rectangular reference frequency for the PLL. This is done by cutting out the amplitude fluctuation components on the synthesized sine wave of the DDS.
The above conventional method has drawbacks in that an expensive D/A converter and a low pass filter, a limiter, etc are needed, and the system becomes large.